Process for the production of alloy steels



United States Patent 3,262,772 PROCESS FOR THE PRODUCTION OF ALLOYSTEELS Adolf Richter, Munich, Georg Cohnen and Hansgeorg Bauer, Witten(Ruhr), Germany, and Rudi Rinesch, Linz (Danube), Austria, assignors toGussstahlwerk Witten Aktieugesellschaft, Witten (Ruhr), Germany NoDrawing. Filed July 1, 1963, Ser. No. 292,127 Claims priority,application Austria, July 6, 1962, A 5,474/62 8 Claims. (Cl. 75-52) Theinvention relates to a process for the production of alloy steels,especially of high grade steels using the oxygen top blowing process.

Liquid basic pig iron or iron produced in a hot blast cupola furnace isused as the star-ting material for this process. Preferably iron low inmanganese is used, for example iron containing 0.7% manganese, as can bepro; duced in the hot blast cupola furnace.

The main feature of the process according to the invention is that it iscarried out in two phases. In the first phase an intermediate productwith a phosphorus content of less than 0.025% is produced under a highlybasic first slag rich in iron oxide, that is to say a slag with abasicity (CaO/SiO of more than 3. After removal of the slag and additionof alloying elements the second phase of the process is carried out. Inthis phase the blowing of the liquid intermediate product is carried outunder a second slag (about 1 to 4% of slag-forming constituents) with abasicity of about 2. The process can be ended when the desired carboncontent is reached.

In the first phase blowing is conveniently carried out initially withnormal blowing energy (corresponding to a lance distance of about 90 to140 cm. and a pressure of 6 to 10 atmospheres excess pressure) andlater, that is during the second half of the first phase, low blowingenergy may be used (corresponding to a lance distance of 200 to 350 cm.and a pressure of 3 to 8 atmospheres excess pressure).

The second phase of the process can be carried out with a normal blowingenergy (corresponding to a lance distance of about 90 to 140 cm. and apressure of 6 to 10 atmospheres excess pressure) or with increasedblowing energy.

The alloying elements are as mentioned above added to the intermediateproduct before the second blowing phase, conveniently in the form offerro-alloys or high alloy scrap, the percentage of such alloy amountingto up to 50% of the metallic charge already in the furnace. Chemicalheat carriers, such as ferrosilicon or the like, can also be added tothe bath before the second blowing phase.

In detail, the process of invention may, for example, be carried out asfollows:

The fluxes required for forming the highly basic first slag in the firstblowing phase are quantitatively dependent on the crude iron analysis.On average they comprise 3 to lime, 1 to 3% ore or iron-rich slag, and0.5 to 1.0% fluorspar. The lance is adjusted to a normal blowing height(90 to 140 cm.) and the blowing pressure is maintained at 6 toatmospheres excess pressure. In the first seconds of blowing a spot ofhigh temperature forms on the bath surface, the carbon ignites and thereresults a dazzling flame with red smoke.

On the bath surface there is first produced an ironrich slag containingmanganese silicate, which rapidly dissolves part of the lime. Three tosix minutes after the commencement of blowing the silicon is completelytaken up into the slag. The thin-flowing slag begins to foam anddissolves still further the solid lime. Approximately in the fifth toseventh minute of blowing in the first phase the lance is furtherwithdrawn to reduce the blowing energy and thus to inhibit thecombustion of carbon, for example to a distance of 200 to 350 cm.,whereupon the carbon flame collapses and the decarburation speed rapidlyfalls. The refining process now occurs essentially only over the slag.The slag foams; the drops of iron still contained in it are oxidised bythis method of blowing and the iron-oxide content of the slag isincreased. The phosphorus content falls to below 0.025% during thisphase, while the carbon content is still about 1% below the carboncontent of the initial crude iron. The bath temperature may be 1400 to1500 C. The first blowing phase is completed after about 10 to 12minutes, when the supply of oxygen is shut off and the lance iswithdrawn upwards. The bath and the foaming slag rich in iron oxidereact further with each other. By slowly tilting the crucible the slagis allowed to run off. The boiling of the bath and foaming of the slaggradually subside. The total iron oxide content of the slag, which aftercompletion of the first blowing phase amounts to more than 30%,decreases in the tilted crucible to 10 to 15%.

At the end of the period during which the crucible is tilted, whichamounts to about 30 minutes, chemical heat carriers, such asferrosilicon are added to-the bath. Then the alloying elements areadded, alloyed with iron, in such amounts that in the finished steel thedesired content of alloying elements is present. Instead of ferroalloys,high-alloy scrap can be charged for this purpose, in which case theeconomics of the process are improved by the utilisation of the alloyingelements present in the scrap.

Before the beginning of the second blowing phase further slag-formingconstitutents are added for the formation of the second slag. If, asmentioned above, heat ca rriers (FeSi) are also added, the lime chargemust be balanced with the amount of silicon added; the basicity of thenew slag (CaO/SiO should amount to about 2.

After addition of the alloying elements and the slagforming ingredientsthe second blowing phase begins. The melt ignites immediately uponcommencement of the supply of oxygen. The heat carriers added (generallysilicon) burn away first on account of their afiinity for oxygen and inthis way promptly raise the temperature of the liquid bath. Ifferro-alloys are added to in troduce the alloying elements, theydissolve in the course of the first five minutes of blowing; if scrap isadded the speed of dissolution is dependent upon the nature of thepieces. If the scrap is in the form of heavy pieces the dissolution timecan amount to as much as 12 minutes.

Since the phosphorus has already been removed during the first blowingphase and the necessary final temperature can be controlled by theaddition of heat carriers, the blowing can be ended at any desiredcarbon content by shutting otf the supply of oxygen. The amount of slagas well as its chemical composition and the high final temperatures ofblowing (over l'680 C.) cause only a small loss of oxygen-sensitivealloying elements. Nickel and molybdenum do not enter the slag. Theyield of chromium and manganese lies between and After completion ofblowing reducing agents, such as aluminum granules, calcium carbide,ferrosilicon and calcium silicide powder and the like can be added tothe slag, whereupon a back reduction of alloying elements from the slaginto the bath occurs.

Deoxidation agents can likewise be added to the bath after completion ofblowing. In this way the quality of the steel can be improved andprecise alloying corrections can be carried out.

If the best quality steel is required with the lowest possible sulphurcontent, a portion of the second slag can be drawn off, powdered limecan be added and this canbe liquefied by means of fluorspar. The amountof these additional slag-forming constituents amounts to about 0.5 to1.5%. This slag is heated by means of gas or oil burners and maintainedin a reducing condition by the addition of reducing agents, such ascalcium carbide, coke, calcium silicide and the like. In general a slagis thus obtained which, like the electric arc furnace refining slag,possesses a strongly desulphurising and deoxidising action.Simultaneously the bath is killed with the known deoxidising agents inthe crucible. In this way it is possible to reduce the sulphur contentto about 0.005%, and carry out corrective additions of alloying elementswithout loss and with great precision, as well as to achieve a qualityof steel equivalent to the electric furnace steel.

After the slag reduction operations, bath deoxidation and correctiveaddition of alloying elements, the melt is tapped and poured into amould once the necessary casting temperature is reached.

We claim:

1. A process for the production of alloy steel from crude iron by theoxygen top blowing process which comprises the steps of:

(1) blowing in a. first phase to form an intermediate product with aphosphorus content of less than 0.025% under a highly basic first slagwhich is rich in iron oxide and has a basicity of more than 3,

(2) removing the slag,

(3) adding alloying elements,

(4) blowing in a second phase under a second slag in which theslag-forming constituents comprise from about 1 to about 4% and whichhas a basicity of about 2, and,

(5) after completion of said second blowing phase, partially removingthe resultant slag and adding fresh slag-forming constituents,liquefying the slag by external heating and maintaining it in a reducingcondition by the addition of reducing agents, so that a reducing slagsimilar to that in an electric are refining slag results, whereby thequality of the steel is improved and the sulphur content of the steel isreduced to 0.005%.

2. A process according to claim 1 in which blowing in said first phaseis carried out initially using a normal blowing energy corresponding toa lance distance of from about 90 to about 140 cm. and a pressure of 6to 10 atmospheres excess pressure and subsequently using a reducedblowing energy corresponding to a lance distance of from about 200 toabout 300 cm. and a pressure of 3 to 8 atmospheres excess pressure.

3. A process according to claim 1.in which blowing in said second phaseis carried out using at least a normal blowing energy corresponding to alance distance of from about 90 to about 140 cm. and a pressure of 6 to10 atmospheres.

4. A process according to claim 1 in which said alloying elements areintroduced in the form of ferro-alloys or high-alloy steel scrap, thepercentage of alloy added amounting to up to of the metallic charge.

5. A process according to claim 1 in which chemical heat carriers areadded to said intermediate product before said second blowing phase.

6. A process according to claim 1 in which after completion of saidsecond blowing phase reducing agents are added to the slag to cause aback reduction of alloying elements from the slag into said resultantsteel.

7. A process according to claim 1 in which deoxidising agents are addedto said resultant steel after completion of said second'blowing phase.

8. A process according to claim 1, including terminating the blowing inthe second phase as soon as the desired carbon content is attained inthe resultant steel.

References Cited by the Examiner UNITED STATES PATENTS 2,303,064 11/1942Perrin 46 2,557,458 6/1951 Ogan 75 51 2,815,275 12/1957 Richter 75522,853,377 9/1958 Kalling et al 7560 2,893,861 7/1959 Rinesch 75523,004,847 10/1961 Lambert et a1. 7560 FOREIGN PATENTS 858,377 l/1961Great Britain.

BENJAMIN HENKIN, Primary Examiner.

1. A PROCESS FOR THE PRODUCTION OF ALLOY STEEL FROM CRUDE IRON BY THEOXYGEN TOP BLOWING PROCESS WHICH COMPRISES THE STEPS OF: (1) BLOWING INA FIRST PHASE TO FORM AN INTERMEDIATE PRODUCT WITH PHOSPHORUS CONTENT OFLESS THAN 0.025% UNDER A HIGHLY BASIC FIRST SLAG WHICH IS RICH IN IRONOXIDE AND HAS A BASICITY OF MORE THAN 3, (2) REMOVING THE SLAG, (3)ADDING ALLOYING ELEMENTS, (4) BLOWING IN A SECOND PHASE UNDER A SECONDSLAGA IN WHICH THE SLAG-FORMING CONSTITUENTS COMPRISES FROM ABOUT 1 TOABOUT 4% AND WHICH HAS A BASICITY OF ABOUT 2, AND (5) AFTER COMPLETIONOF SAID SECOND BLOWING PHASE, PARTIALLY REMOVING THE RESULTANT SLAG ANDADDING FRESH SLAG FORMING CONSTITUENTS, LIQUIDFYING THE SLAG BY EXTERNALHEATING AND MAINTAINING IT IN A REDUCING CONDITION BY THE ADDITION OFREDUCING AGENTS, SO THAT A REDUCING SLAG SIMILAR TO THAT IN AN ELECTRICARC REFINING SLAG RESULTS, WHEREBY THE QUALITY OF THE STEEL IS IMPROVEDAND THE SULPHUR CONTENT OF THE STEEL IS REDUCED TO 0.0005%.